FIG. 3(a) is a perspective view illustrating a structure of a prior art AlGaAs series semiconductor laser described in "High Power 780 nm AlGaAs Quantum Well Lasers And Their Reliable Operation", IEEE Journal of Quantum Electronics, volume 27, Number 6, 1991, pages 1544-1549. FIG. 3(b) is an enlarged view of a portion IIIb of the structure shown in FIG. 3(a). An n type GaAs substrate 16 has opposite front and rear surfaces. An n type GaAs buffer layer 2b is disposed on the n type GaAs substrate 1b. A lower cladding layer 3b comprising n type Al.sub.x Ga.sub.1-x As (x.about.0.5) is disposed on the buffer layer 2b. A triple-quantum well active layer 4c comprising three Al.sub.y Ga.sub.1-y As (y.about.0.1) well layers 7c, two Al.sub.z Ga.sub.1-z As (z.about.0.3) barrier layers 8c, and two Al.sub.r Ga.sub.1-r As (r.about.0.3) guide layers 9c are disposed on the lower cladding layer 3b. An upper cladding layer 5b comprising p type Al.sub.s Ga.sub.1-s As (s.about.0.5) and having a stripe-shaped ridge is disposed on the triple-quantum well active layer 4c. A p type GaAs cap layer 6b is disposed on the top of the ridge of the upper cladding layer 5b. A current blocking layer 10b comprising n type GaAs is disposed on the upper cladding layer 5b contacting opposite sides of the ridge. A p type GaAs contact layer 11b is disposed on the p type GaAs cap layer 6b and on the n type GaAs current blocking layer 10b. An electrode 13 is disposed on the rear surface of the n type GaAs substrate 1b and an electrode 12 is disposed on the p type GaAs contact layer 11b.
FIGS. 4(a)-4(c) show process steps for fabricating the prior art AlGaAs series semiconductor laser. As shown in FIG. 4(a), the layers from the n type GaAs buffer layer 2b to the p type GaAs cap layer 6b are successively grown on the n type GaAs substrate 1b by MBE (Molecular Beam Epitaxy) or MOCVD (Metal Organic Chemical Vapor Deposition).
A silicon nitride (SIN) film 14 is formed on the p type GaAs cap layer 6b and patterned by photolithography to leave a stripe-shaped portion in the center region on the cap layer 6b. Thereafter, using the stripe-shaped SiN film as a mask, the p type GaAs cap layer 6b and the p type Al.sub.s Ga.sub.1-s As (s.about.0.5) upper cladding layer 5b are selectively etched, thereby forming a ridge 20 shown in FIG. 4(b).
Thereafter, an n type GaAs current blocking layer 10b is selectively grown. After removal of the SiN film, a p type GaAs contact layer 11b is grown over the entire surface, completing the structure shown in FIG. 4(c).
The respective layers included in the semiconductor laser have about the same lattice constant and lattice-match with each other. On the other hand, it is described in E. Yablonovitch and E. O. Kane, "Band Structure Engineering Of Semiconductor Lasers For Optical Communications", Journal of Lightwave Technology, Volume 6, page 1292, 1988, that, theoretically, laser characteristics are improved when a strain is introduced in the structure by employing an active layer having a lattice constant different from the lattice constants of other layers.
The band structure of a III-V compound semiconductor used in semiconductor lasers is shown in FIG. 5(a). In the Figure, the upper curve represents the conduction band and the lower curve represents the valence band. Characters F.sub.c and F.sub.v represent quasi-Fermi levels and a character E.sub.g represents a forbidden band gap. In this case, the effective mass of a hole in the valence band is much larger than the effective mass of an electron in the conduction band. Therefore, in the band structure of FIG. 5(a), the state density in the vicinity of the top of the valence band is high. In this case, in order to make a laser oscillate, a carrier density that is higher than the carrier density obtained in a band structure where the effective mass of a hole in the valence band is approximately equal to the effective mass of an electron in the conduction band, as shown in FIG. 5(b), is required. Accordingly, in order to reduce the threshold current for laser oscillation, the energy band structure shown in FIG. 5(b) has an advantage over the energy band structure shown in FIG. 5(a). On the other hand, various processes such as inter-valence band absorption and Auger recombination are obstacles to a reduction in the threshold current. The actual valence band comprises a heavy hole band, a light hole band, and a spin orbit split-off band (in FIGS. 6, 7, and 8, referred to as an HH band, an LH band, and an SO band, respectively). In FIGS. 5(a) and 5(b), only the HH band mainly contributing to optical transition is depicted. Inter-valence band absorption is absorption of light due to combination of an electron in the SO band with a hole in the HH band, as shown in FIG. 6. Further, the Auger recombination process comprises two successive steps in which energy due to recombination of an electron in the conduction band (CB) and a hole in the HH band is not emitted as light but dissipated by exciting an electron from the SO band, leaving a hole at the same place. However, if the energy band structure shown in FIG. 5(b) having a small effective hole mass in the valence band is realized, both the inter-valence band absorption and the Auger recombination are suppressed because of the reduction in the carrier density, whereby a reduction in the threshold current and an increase in the output power are promoted. It is clarified theoretically that the band structure shown in FIG. 5(b) can be realized by introducing a compressive strain. In other words, when a compressive strain is introduced into the III-V compound semiconductor, degeneracy in the valence band is eliminated and the top of the HH band providing a valence band of the highest energy (HH1 in FIG. 8) is higher than the HH band in a case where no strain is introduced (dotted line in the Figure), whereby an energy band structure close to FIG. 5(b) is obtained.
There have been various attempts at improving the characteristics of semiconductor lasers employing the compressive strain. First of all, an example of a strained quantum well structure semiconductor laser fabricated on an InP substrate (Ken Kamijyo, Hideaki Horikawa, "Improvement In Characteristics Of A Semiconductor Laser In A Strained Quantum Well Structure", Journal of Application Physics, Volume 62, page 134, 1993) will be described. This laser produces light having a wavelength of 1.48 .mu.m directed to applications in optical communications. FIG. 9(b) shows the laser structure. An active layer comprises an InGaAsP multi-quantum well structure shown in FIG. 9(a) and has a lattice constant larger than the lattice constant of InP, the material of the substrate and a cladding layer, whereby a compressive strain is introduced. The saturation output power of this laser is higher than that of a multi-quantum well structure laser having no strain.
Next, an example of a strained quantum well structure semiconductor laser fabricated on a GaAs substrate (C. A. Wang et al., "AlInGaAs--AlGaAs Strained Single-Quantum-Well Diode Lasers," Volume 3, page 4, 1991) will be described. For applications in optical disk devices, a high-output power laser producing light with a wavelength shorter than 1.48 .mu.m or 1.55 .mu.m, which is used in optical communication, is required. An AlGaAs series laser fabricated on a GaAs substrate is such a short-wavelength laser. In this example, an active layer comprises a single quantum well structure in which a compressive strain is introduced into the well layer. More specifically, the active layer comprises a guide layer comprising Al.sub.y Ga.sub.1-y As (y=0.3.about.0.7) and a well layer comprising Al.sub.y In.sub.x Ga.sub.1-x-y As (x=0.12.about.0.14, y=0.05.about.0.17). Thereby, operation in a short wavelength band of 785-890 nm is realized. However, the threshold current and differential quantum efficiency of this prior art laser are approximately equal to those of the AlGaAs series laser. In addition, in a reliability test, the reliability of the prior art AlGaAs series laser was not exceeded.
When a compressive strain is introduced, the energy level of the light hole band is lower than the energy level of the heavy hole band. However, when a tensile strain is introduced, the energy level of the light hole band is higher than the energy level of the heavy hole band, and the contribution of the light hole band to the optical transition becomes large (Tsukuru Ohtoshi, "Strained Quantum Well Laser", Applied Physics, Volume 59, page 1193, 1990). This effect is illustrated in FIG. 10.
With respect to the oscillation mode in the optical transition of a semiconductor laser, the light hole band corresponds to the TM mode and the heavy hole band corresponds to the TE mode. There is a semiconductor laser light amplifier in which an amplification independent of polarization is realized by introducing a tensile strain into an active layer to control the optical transitions in the TE mode and the TM mode. This amplifier is disclosed in the article by Takeshi Kamijou and Hideaki Horikawa. The structure of the amplifier is the same as that shown in FIG. 9(b). In this structure, InP is employed for the substrate and the composition ratios of respective components of InGaAsP in an active layer are controlled to introduce a tensile strain into a quantum well layer. As is apparent from signal gain characteristics shown in FIG. 11, an amplification independent of polarization is realized with a tensile strain of 0.2%. On the other hand, an AlGaAs series semiconductor laser in which a tensile strain is introduced into an active layer is not yet known.
There are three modes of characteristic deterioration in AlGaAs series lasers, i.e., rapid deterioration, gradual deterioration, and sudden i.e., catastrophic, deterioration, sometimes called sudden death, and a main cause thereof is dark line deterioration. A dark line defect produces a non-emission region in the vicinity of dislocations that are grown in an oscillation region of an active layer. Therefore, in order to suppress dark line defect, it is required to suppress proliferation of dislocations from layers other than the active layer and from the substrate to the active layer. It is well known that employing a material including In, such as InGaAs, for the active layer is effective for this purpose. In addition, it is preferable to use a substrate including a reduced density of dislocations. With respect to an LEC (Liquid Encapsulated Czochralski) GaAs substrate, it is well known that the dislocation density can be reduced by two to three orders of magnitude by adding In. However, the In concentration in this case is below 1%.
In the prior art lattice matching AlGaAs series lasers, the energy band structure is an obstacle to a reduction in the threshold current and an increase in the output power. Further, the dark line deterioration reduces reliability.
In order to solve the above-described problems, a strained quantum well structure laser including an InGaAs active layer has been reported and provides reduced threshold current and improved reliability. However, since InGaAs has a narrower energy band gap than AlGaAs, the oscillation wavelength is longer than that of a laser including an AlGaAs active layer. Therefore, this laser is not preferable in view of the reduction in the wavelength of the light produced. Although the article by C. A. Wang et al. suggests an AlInGaAs active layer in order to achieve a short wavelength while preserving the quality of an active layer including In, neither the laser characteristics nor reliability is improved as described above. These two examples introduce a compressive strain into the active layer.
In a case where a compressive strain is introduced, a reduction in effective mass of holes is intended by increasing the curvature in the vicinity of the top of the heavy hole band. However, when a tensile strain is introduced, because the light hole band originally having a small effective hole mass has the highest energy level in the valence band and contributes to optical transitions, it is expected that effects of reducing the threshold current and increasing the output power are the same as in a case where a compressive strain is introduced. However, there is no example that positively utilizes the effects obtained by the introduction of tensile strain. There is only an example in which the optical transitions of the TE mode corresponding to the heavy hole band and the TM mode corresponding to the light hole band are controlled by the semiconductor laser light amplifier with an InGaAsP active layer on the InP substrate to realize an independence of gain and polarization, as described by Ken Kamijou and Hideaki Horikawa.
As means for introducing a tensile strain in an active layer in an AlGaAs series laser, a material containing P as the Group V element has been thought of. However, it is difficult to control accurately both As and P simultaneously in epitaxial growth.